(1) Field of the Invention
The present invention relates to nondestructive testing of underwater sound transducers and more particularly to an apparatus and method for disclosing the presence or absence of loose parts and/or faulty bonds in hydrophones by projecting a pulsed dual-frequency sound field and measuring the response of the test transducer for nonlinearities indicated by the presence of an erratic difference-frequency signal.
(2) Description of the Prior Art
Hydrophones are considered to be generally linear devices. This linearity is relied upon when analyzing data received by operational hydrophone arrays or when making many other acoustic test measurements. While slight deviations in linearity can usually be tolerated, manufacturing defects such as loose parts and/or defective boot bonds may introduce significant nonlinearities during system operation or test resulting in erroneous data. Detection of boot bond defects to date has been costly in that production samples must be destructively tested and also has not been totally accurate in that untested units may still be defective. Present nondestructive test methods, such as X-ray inspection and ultrasonic flaw detection, have been unsuccessful in detecting the very thin voids which can occur within a bond. Loose interior parts can be detected by the presence of harmonics in the signal radiated by the transducer, but transducer boot bonds can presently only be checked by costly destructuve methods. What is needed is a relatively inexpensive nondestructive testing system which can be used to inspect all manufactured units for both types of defects.